In optical communications, it is difficult and often costly to make the connection between laser diodes and fibers, particularly single-mode fibers. In order to achieve efficient optical coupling, the laser must be aligned to the fiber to tight tolerances; typically .+-.1 to 2 .mu.m for single-mode fibers and .+-.10 .mu.m for multimode fibers.
Methods of laser-to-fiber alignment may be divided according to two criteria:
1. Piece-by-piece vs. batch. In piece-by-piece methods, an individual laser die, usually pre-packaged in a TO can, is aligned to a single fiber, or to the center of a bore which later accepts a fiber centered in a precision ferrule. By contrast, in batch methods, an array of lasers is aligned to an array of fibers. This succeeds to the extent that each array is straight and the center-to-center spacings match. PA1 2. Active vs. passive. In active alignment methods, the laser is energized to produce light; whatever portion of this light successfully emerges from the back end of the fiber is monitored by a photodetector. Thus, the best-aligned position is determined explicitly by moving the energized laser with respect to the fiber to maximize the photodetected signal. By contrast, in passive alignment methods, the best-aligned position is determined implicitly by microscopic visual alignment of certain geometric features assumed to have known relationships to the optical centerlines.
Most prior-art alignment schemes are piece-by-piece and active. These schemes are inherently expensive because each part is handled individually, each fiber is individually polished, and each alignment is separately performed. Moreover, arrays of interconnects which are needed to achieve high-speed data rates via multiple fibers are bulky and inconvenient if built from piece-made packages.
Several prior-art alignment schemes are batch and passive. Batch alignment is advantageous because it shares the cost of handling, polishing, and alignment among the many elements of an array. However, the advantages of passive alignment are questionable: the schemes require costly precision jigging for "dead reckoning," and impose tight tolerances on the location of fiducials or other geometric features which must be precisely located with respect to the optical centerlines. Moreover, the method may require custom laser arrays having special fiducials, and imposes stringent tolerances on the V-grooves which hold the fibers. Both of these requirements are costly.